This invention generally relates to friction materials which are used in torque transmitting apparatus having a friction facing material operatively engageble with an opposing surface in the presence of a transmission fluid or oil. More particularly, the invention relates to a friction facing material comprising metallic particles in the form of a porous body having excellent friction properties and durability, and to the method for making such materials.
The torque transmitting apparatus referred to above may comprise, for example, clutch and brake assemblies having a friction disc and an opposed plate. The friction facing material, in the form of a grooved or ungrooved disc, or disc segments, is secured to a metallic core to provide a friction or torque transmitting surface thereon. The opposing plate provides a cooperating surface which operatively engages the friction surface for torque transmission. A plurality of discs having friction facing material on opposed surfaces are normally interleaved with a plurality of opposing plates to provide a multiple disc clutch. Torque transmission is regulated by closing means which control the axial proximity of the adjacent discs and plates.
The disc and opposing plate may extend to a reservoir of transmission fluid, or the fluid may be delivered to the disc under pressure from such reservoir or from a remote reservoir. The fluid serves to cool the apparatus by dissipating the heat energy resulting from torque transmission, which is referred to as wet operation of the unit. The fluid may also serve to transmit torque by the shearing of films of fluid between adjacent discs and plates, as well as to dissipate heat, which is referred to as hydroviscous operation of the apparatus.
The heavier duty torque-transmitting apparatus and applications of concern herein are of the type encountered in large road vehicles, such as buses and trucks as well as off-the-highway and construction vehicles. In order to meet the torque loading requirements of such applications, friction facing materials composed of graphite in a powdered metal matrix pressed using high pressures to form a green compact and then sintered at high temperatures and pressures have been developed.
Another type of friction material involves the use of relatively high proportions of abrasive or ceramic materials with minimal proportions of graphite in a powdered metal matrix. These loose mixtures are sprinkled onto a metallic core and then sintered in place to form a facing and to bond the facing to the metallic core. While these types of friction facing materials exhibit a relatively high dynamic coefficient of friction they are limited to less severe applications due to their limitations in energy absorption rates.
The torque transmission characteristics are determined by a number of factors, including the particular transmission fluid and the friction facing material as well as the nature of the cooperating opposing plate surface. The resiliency of the friction facing is a major contributing factor to the torque transmission characteristics in that more resilient friction facings conform better to the opposing plate surface thereby providing more uniform energy absorption over the area of the friction facing. More resilient friction facing generally can tolerate higher energy absorption rates due to the more uniform absorption of energy over the area of the facing. Less resilient friction facings are limited in their rate of energy absorption by this same factor. Heretofore, metallic based friction facings have exhibited low levels of resiliency and as a result have been limited in their rate of energy absorption. Metallic based friction facings manufactured utilizing high pressures to form a green compact are limited in resiliency due to the dense structure of the friction facing obtained with this method. Metallic based friction facings manufactured using the sprinkling process are limited in resiliency due to the coarse rigid metallic matrix inherent with these type of friction facings.